1. Field of the Invention
This invention relates to a thermal transfer type color printer in which plural ink ribbons having different color components are arranged and interposed between a printing head and a printing medium and the printing head heats the ink ribbons so that ink of each of the ink ribbons is melted and transferred to the printing medium and a color image is formed.
2. Description of the Related Art
FIG. 1 schematically shows a conventional thermal transfer type color printer disclosed in Jap. Pat. Appln. KOKAI Publication No. 59-188452.
As shown in FIG. 1, this color printer has print units 101, 102, 103 and 104 for yellow (Y), magenta (M), cyan (C) and black (K) sequentially arranged on a conveying path 105 for conveying a paper sheet 106.
For example, the print unit 101 for yellow has a thermal line head 101-1, an ink ribbon mechanism and a platen roller 101-4 for transfer. The ink ribbon mechanism has a supplying roller 101-3A and a winding roller 101-3B as a pair. The supplying roller 101-3A supplies a yellow ink ribbon 101-2 including yellow ink onto a heating face of a heating resistor forming the thermal line head 101-1.
The other print units 102 to 104 also have a structure similar to that of the yellow print unit except that the ink ribbons respectively include magenta, cyan and black ink. Accordingly, a more detailed description of the print units 102 to 104 is omitted here.
The paper sheet 106 is conveyed from the yellow print unit 101 to the black print unit 104 along a conveying path 105 by a first feed roller pair 107 and a second feed roller pair 108. In each of the print units 101 to 104, the paper sheet 106 passes through portions between the respective ink ribbons 101-2 to 104-2 and the respective platen rollers 101-4 to 104-4.
At a printing time, while the paper sheet 106 is conveyed from the yellow print unit 101 to the black print unit 104, a yellow image is first printed on the paper sheet 106 by the yellow print unit 101. When this printed portion reaches the heating face of each of the thermal line heads 102-1 to 104-1 of the other respective print units 102 to 104, respective color images are synchronously overlapped and printed sequentially on the paper sheet. At this time, the plural inks are overlapped and mixed with each other so that an image having a predetermined hue is printed.
Thus, in the thermal transfer type color recording method of a 3 to 4 head system having one thermal line head every one color, an image can be printed at high speed since no paper sheet 106 is repeatedly reciprocated every print of one color as in one head system.
In the thermal transfer type color printer, when a paper sheet having a low surface smoothness, i.e., a paper sheet having a rough surface is used, an amount of ink permeating recessed portions of this paper sheet is insufficient so that it is difficult to transfer the ink to the paper sheet and there is a fear of generation of a whitish extracting state in which no characters are printed. Therefore, there is a problem of a reduction in printing quality.
Two methods are considered to solve this problem.
A first method is a method described in NIKKEI ELECTRONICS 1995. 7. 17, No. 640, p.99. In accordance with this description, the first method uses a serial type thermal head constructed such that ink is printed onto a paper sheet while a cartridge having a thermal head is moved in a main scanning direction perpendicular to a conveying direction of the paper sheet. Ink having a high melting viscosity and including resin is used and heated and melted. An ink ribbon and a recording medium are separated from each other before this ink is solidified. Thus, as shown in FIG. 2, the ink 113 is transferred to a recessed portion 112 on a surface 111 of the recording medium in a bridging shape.
As shown in FIG. 3, the used ink ribbon has a separating layer 122 and a resin-including ink layer 123. The separating layer 122 is formed on a base film layer 121 and has 1.3 .mu.m in thickness. The resin-including ink layer 123 is formed on this separating layer 122 and has 1.5 .mu.m in thickness. This separating layer 122 is formed by a material having a low melting viscosity and is completely melted at a softening temperature of the ink layer 123 so that the separating layer 122 has almost no adhesive force. Therefore, the separating layer 122 acts as a layer for easily separating the ink layer 123 from the base film layer 121. A back coat layer 124 is formed outside the above base film layer 121.
The resin-including ink is formed by dispersing a pigment to thermoplastic resin and can hold a high viscosity even at a temperature such as about 100.degree. C. In contrast to this, wax-including ink including wax, etc. has a low viscosity so that the wax-including ink is almost liquefied at a temperature such as about 100.degree. C.
A second method is a method in which ink easily permeates a paper sheet until a recessed portion thereof by using the wax-including ink of a low melting viscosity.
However, in the conventional thermal transfer type color printer having the plural print units continuously arranged as shown in FIG. 1, it is necessary to raise a heating resistor of the thermal line head to a high temperature for a very short time and stably melt ink in a wide range and transfer this ink to the paper sheet when a color print is made at high speed. Therefore, in the thermal transfer type color printer of the conventional 3 to 4 head system, for example, a printing condition is basically different from that in the printer of a 1 head system using a serial type thermal head so that it is difficult to adopt the above-mentioned two methods.
Namely, when the above-mentioned first method is applied to the thermal transfer type color printer of the 3 to 4 head system, there is a problem of generation of a phenomenon of leaving ink on the base film at a transfer time, etc. For example, the printer using the serial type head uses ink ribbons of four colors or four ink cartridges including a separating layer having a low melting viscosity and a resin-including ink layer having a high melting viscosity. An entire paper sheet is first printed in yellow (Y) and is next repeatedly printed four times in a sequential order of magenta (M), cyan (C) and black (K) so that a color print is realized by overlapping the colors. When the colors are overlapped, ink as a base previously printed is already solidified. The wax-including ribbon used in the second method shows characteristics in which viscosity is suddenly reduced at a certain temperature. However, the resin-including ink shows characteristics in which viscosity is gradually reduced with a rise in temperature. Accordingly, the printer using the conventional serial type head has a sufficient time margin while the colors are overlapped. Therefore, the ink as a base previously printed is sufficiently cooled and solidified.
In contrast to this, in the printer of a 3 to 4 head system as shown in FIG. 1, the paper sheet sequentially passes through the plural thermal units continuously arranged at high speed. In this passage, a color overlapping interval of each of the thermal units is short so that no ink of a color as a base previously printed is cooled. Accordingly, the next ink is overlapped with the previously printed ink in a state in which no previously printed ink is sufficiently solidified.
Similar to the case of directly transferring ink onto the paper sheet, when the next color ink is simply transferred onto the ink not sufficiently solidified, there is a fear of generation of a phenomenon of leaving the ink on the base film when an ink ribbon is separated from the paper sheet. In the case of a color thermal transfer record, color overlapping is one of the most difficult processes. In particular, a surface state of a printed object as a base exerts a great influence on a printing quality of the printed object so that a recording method not influenced by the base is desirable.
Further, a load applied to the ink ribbon by a thermal head in the printer using the serial type head is greatly different from that in the printer of the 3 to 4 head system.
Namely, in the printer using the serial type head, a large load can be applied to the ink ribbon in comparison with the thermal line head. Therefore, the ink ribbon can sufficiently come in press contact with the paper sheet by this load so that the resin-including ink of a high melting viscosity can be transferred to the paper sheet.
However, when the large load is applied in the case of the thermal line head, wrinkles are caused in the paper sheet and the ink ribbon and a color shift between the respective units is increased. Therefore, only a small load is applied to the ink ribbon in comparison with the serial type head. When no ink ribbon can sufficiently come in press contact with the paper sheet, an untransfer portion of ink onto the paper sheet is caused when the resin-including ink of a high melting viscosity is used. Accordingly, there is a fear of generation of a whitish extracting state wherein no characters are printed on the paper sheet.
In particular, when a third color is printed in the case of a color record, it is necessary to transfer ink for a short time. However, there are irregularities on the surface of a recording medium and there are also irregularities of thicknesses of two color inks previously printed. Accordingly, the end portions of a transferred ink image are not sharply printed and ink is insufficiently fixed onto the paper sheet so that there is a reduction in printing quality.
Further, when the second method is applied to the printer of the 3 to 4 head system and characters are printed at high speed, the amount of ink permeating recessed portions of the recording medium is insufficient even when the ink is melted. Accordingly, the whitish extracting state can not be prevented. A sufficient permeating amount of the ink permeating the recessed portions of the recording medium on its surface is required to prevent the whitish extraction in this second method. Therefore, the second method is effective in a monochromatic printer, but there is a fear of insufficiency of the permeating amount of ink in second and third overlapping prints in a high speed print in the color printer.
As mentioned above, in the thermal transfer type color printer using the conventional thermal line head, it is difficult to print a color image having a high quality at high speed due to the influences of a surface state of the paper sheet and a transfer state of ink previously printed.